In the field of high pressure sodium lamps, the best known configuration includes a tubular ceramic arc tube disposed within an evacuated glass envelope. The arc tube is filled or dosed with a rare gas and an excessive amount of sodium and mercury. In other words, the arc tube has a saturated sodium fill because of the well known tendency whereby large amounts of sodium are lost during operation of the lamp. Moreover, such saturated type high pressure sodium lamps leave much to be desired due to the undesired variations in color rendition and operational voltages which are encountered.
In order to eliminate or at least reduce the above-mentioned undesirable characteristics, the so-called unsaturated vapor type high pressure sodium lamp was developed. Herein the construction is somewhat similar to the saturated type lamp except that the dosage of sodium, in particular, is greatly reduced. Moreover, it was found that such a reduction in sodium content was possible so long as provision was made for absorbing the excess oxygen which undesirably accompanied the fill or rare gas which was dispensed into the arc tube.
One of the techniques for effecting this absorption of undesired oxygen within the arc tube is to enclose an oxygen-absorbing getter therein along with the usual dosage of sodium, mercury and a rare gas. One such structure is set forth and described in a concurrently filed application U.S. Ser. No. 473,895, assigned to the Assignee of the present application.
As set forth therein, a getter material is located within the arc tube and in contact with the gases therein. Thus, any undesired oxygen within the arc tube occurring during the sodium or mercury vaporization process or accompanying the admitted rare gas is absorbed by the getter material and compound reformation is inhibited.
However, it has been found that even though the above-described technique has provided great improvement over prior known unsaturated vapor high pressure sodium lamp construction and fabrication, there are still areas which leave something to be desired. More specifically, it has been found that direct contact between the enclosed getter material and the hottest portions of the tubular ceramic envelope of the arc tube tend to cause what appears to be a chemical reaction therebetween and an undesired darkened area of the ceramic envelope. Obviously, arc tube darkening is not a desirable condition.